The disclosure relates to a method and an apparatus for controlling the operational performance of an internal combustion engine within a predetermined domain. The method and the apparatus relate to changes in the fuel-air ratio provided to the engine or the amount of recycled exhaust gas in dependence on dispersion of the cyclic variations of the mean combustion chamber pressure during intervals which are synchronous with the engine r.p.m. (dynamic stability control). The invention relates to providing a signal related to the non-uniform operation of the engine and a corresponding electrical signal related to a uniform engine operation and it further relates to determining the phase relations of these two signals as a measure for the control variable. The actual value is compared with the set-point value and a comparison signal is fed to an integrating final control element as described in detail in the parent application Ser. No. 597,404. It is the purpose of the control system described in the parent application Ser. No. 597,404 to permit operating internal combustion engines in a domain of operation in which toxic components of the exhaust gases are as low as possible and/or the fuel consumption is as low as possible so as to meet the evermore rigorous regulations regarding exhaust gas constituents and also to accomodate to the universal fuel shortage.
Thus, the parent application proposes a system that makes it possible to operate an internal combustion engine with a relatively lean fuel-air mixture, i.e., the engine is operated at a setting in the direction of a leaned-out mixture so that it can operate in a region which produces relatively harmless exhaust gas and low fuel consumption. For this purpose, it is very important to determine the desired operational point defining the lean running limit of the engine as exactly as possible and to operate the control system at that point. Since the dynamic instability of an internal combustion engine increases when the operation takes place further away from the stoichiometric ratio (air number .lambda..apprxeq.1), the system of the parent application provides measuring the fluctuations in the mean cylinder pressure by going back to the torque fluctuations which are caused by pressure fluctuations in the cylinders. These torque fluctuations are measured with an r.p.m. synchronous signal derived from crankshaft rotation corresponding to the changes in angular acceleration and this signal is used to generate a control signal.
For this purpose, the apparatus of the parent application provides a crankshaft marker and an inductive transducer which generates a signal when the marker passes it, and this signal is fed to a pulse shaping stage and hence to a phase comparator circuit. At the same time, the signal is integrated twice and fed to a voltage controlled oscillator whose output signal is fed to the second input of the phase comparator. This hook-up is a so-called PLL loop (Phase locked loop).
It is the purpose of such a circuit to measure the period of time elapsing between the passage of crankshaft markers and thus to create an electronically simulated comparison system which runs at the same basic r.p.m. of the engine crankshaft but does not have the cyclic variations whose measurement finally results in obtaining the control signal. In other words, one of the two inputs of the comparison circuit receives a signal which corresponds to a uniformly rotating engine, whereas the other input receives a signal whose value is related to the either positive or negative engine accelerations and which comes from the preceding pulse shaping stage.
At the output of the second integrating circuit, the signal, which now corresponds to the crankshaft r.p.m., is transformed into a proportional current and serves to charge a capacitor, and the charging process is controlled by the output signal from the phase comparator which corresponds to changes in the crankshaft period of revolution. Thus, the capacitor is charged in proportion to the error in the period of rotation multiplied by the r.p.m., and the voltage on the capacitor is proportional to these values. In particular, the circuit used in the apparatus of the parent application is shown in FIG. 3 thereof. The signal from the capacitor which was charged in proportion to r.p.m. is fed to the non-inverting input of an operational amplifier, connected as a comparator, whose other input receives a nominal, or set-point, value. The output of the comparing operational amplifier is fed to a bistable multivibrator whose output signal is either a logical 1 or a logical 0. If the output of the bistable multivibrator is a 0, then a subsequent integrating final control element, in the form of an operational amplifier, integrates in the positive direction, i.e., the output voltage of the operational amplifier increases. On the other hand, if the output of the bistable multivibrator is a logical 1, then the actual value is greater than the set-point value and the output signal from the operational amplifier (final control element) goes to 0. The bistable multivibrator is set by a clock pulse which ultimately comes from the output of the phase comparator circuit (more precisely from a subsequent amplifier).
In the known circuit, it is possible that conditions may arise which will cause the control process to become unsatisfactory. Such conditions are, for example, when the engine is operated at an extremely lean operational point and when the supplied fuel quantity is changed in the direction of enrichment (.lambda.-correction) so that the torque of the engine increases and thus indicates a positive acceleration. This positive acceleration is not, in principle, a dynamic instability but rather is a desired change, but the controller senses it as a dynamic instability and thus causes the final control element to perform a further .lambda.-correction in the direction of an enriched mixture. This behavior can cause instability or control oscillations.